A solar cell is a core element of solar-light power generation, which directly transforms solar light into electricity, and it may be basically considered as a diode having a p-n junction. Solar light is transformed into electricity by a solar cell as follows. If solar light is incident to a solar cell, an electron-hole (pair) is generated, and as the generated electrons and holes diffuse, due to the electric field formed at the p-n junction, electrons move to an n layer and holes move to a p layer, thereby generating photoelectromotive force between the p-n junctions. In this way, if a load or system is connected to both terminals of the solar cell, an electric power may flow to generate power.
A general solar cell is configured to have a front surface and a back electrode respectively at front and back surfaces of the solar cell. Since the front electrode is provided to the front surface which is a light-receiving surface, the light-receiving area decreases as much as the area of the front electrode. In order to solve the decrease of the light-receiving area, a back electrode-type solar cell has been proposed. The back electrode-type solar cell maximizes the light-receiving area of the front surface of the solar cell by providing a (+) electrode and a (−) electrode on a back surface of the solar cell.
The back electrode-type solar cell is classified into interdigitated back contact (IBC), point contact type, emitter wrap through (EWT), metal wrap through (MWT) or the like. Among them, the MWT-type solar cell is configured so that, among a grid finger and a bus bar on the front surface, the grid finger remains on the front surface but the bus bar is moved to the back surface, and the grid finger on the front surface and the bus bar on the back surface are connected through a via hole formed through the substrate.
The MWT-type solar cell is configured as follows. As shown in FIG. 1, an emitter layer 102 is provided over the entire surface of a substrate 101, and an anti-reflection film 103 and a front grid electrode 104 are provided on the front surface of the substrate 101. In addition, an n electrode 105 and a p electrode 106 are provided at the back surface of the substrate 101 and electrically connected to the n electrode 105 and the front grid electrode 104 by means of the via hole 108 formed through the substrate 101.
Along with it, in order to prevent an electric short between the emitter layer 102 at the front surface of the substrate 101 and a p+ region at the back surface of the substrate 101 and a short between the n electrode 105 and the p electrode 106, isolating trenches 107 are respectively provided to the front and back surfaces of the substrate 101. The isolating trench 107 is generally formed by means of laser irradiation.
In the conventional MWT-type solar cell configured above, since the isolating trenches 107 are provided respectively to the front and back surfaces of the substrate 101, two laser processes must be performed. In addition, since the isolating trench 107 is provided at the front surface of the substrate 101, the light-receiving area is limited.
In addition, the emitter layer 102 is formed to have a uniform doping concentration, and the doping concentration should be high in order to minimize a contact resistance with the grid electrode 104. However, the emitter layer doped to the light receiving unit with a high concentration increases a recombination loss, and so the short-wavelength optical response characteristic is low.